Psy 8960, Fall ‘06 EPI, Part 2: variants1 Segmentation Partial Fourier Spin echo vs. gradient echo Inversion recovery Long vs. short TE.

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Presentation transcript:

Psy 8960, Fall ‘06 EPI, Part 2: variants1 Segmentation Partial Fourier Spin echo vs. gradient echo Inversion recovery Long vs. short TE

Psy 8960, Fall ‘06 EPI, Part 2: variants2 Last week –Chemical shift Subcutaneous fat resonates at a frequency 3.5 ppm different from water Shift in resonant frequency results in shift in location in image –FLASH-like images: PE direction is impervious; errors show up in read- out direction –EPI-like images: PE direction is the bad guy –Idle curiosity: why does myelin not show up shifted?

Psy 8960, Fall ‘06 EPI, Part 2: variants3 This week Review –Hardware –Pulse sequence diagrams Read vs. phase encode –Pulse sequences FLASH vs. EPI

Psy 8960, Fall ‘06 EPI, Part 2: variants4 Fancy fenders Hardware: everything’s a coil Superconducting coil (“magnet”) Gradient coils (“gradients”) RF coil (“body coil”) RF coil (“head coil”) (from

Psy 8960, Fall ‘06 EPI, Part 2: variants5 Single-shot EPI pulse sequence RF G SS G PE G RO DAC TE

Psy 8960, Fall ‘06 EPI, Part 2: variants6 Spin Echo EPI pulse sequence RF G SS G PE G RO DAC TE/2

Psy 8960, Fall ‘06 EPI, Part 2: variants7 Inversion Recovery Spin Echo EPI RF G SS G PE G RO DAC TE/2

Psy 8960, Fall ‘06 EPI, Part 2: variants8 Inversion Recovery FLASH RF G SS G PE G RO DAC N PE N RO

Psy 8960, Fall ‘06 EPI, Part 2: variants9 Segmented EPI pulse sequence RF G SS G PE G RO DAC TE N PE /N SEG

Psy 8960, Fall ‘06 EPI, Part 2: variants10 Turbo Spin Echo (turbo factor = 4) RF G SS G PE G RO DAC TE?N PE /TF

Psy 8960, Fall ‘06 EPI, Part 2: variants RF G SS G PE G RO DAC 128 … … (32 times) Echo spacing = 0.75ms Pulse sequence diagram: Flip angle = 40 degrees Sequence type: segmented EPI Contrast: T 1 and T 2 * Image resolution: 128 x 128 Segment acq. time: 32 x 0.75ms = 24 ms Image acq. time: 1 sec TE = 20ms (16 th line) N rep = 4 TR = 0.25 s Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Segment acq. time: Image acq. time: ___________

Psy 8960, Fall ‘06 EPI, Part 2: variants12 K-space trajectory and scaled T 2 * decay in gray matter: 128 (complex-valued) points T acq,seg = 24ms TE = 20ms time (ms) MM … … … … 128 lines

Psy 8960, Fall ‘06 EPI, Part 2: variants13 Image:

Psy 8960, Fall ‘06 EPI, Part 2: variants RF G SS G PE G RO DAC 128 … … (96 times) Echo spacing = 0.75ms Pulse sequence diagram: 60 deg. Sequence type: GE EPI (partial Fourier acquisition) Contrast: T 2 * Image resolution: 128 x 128 Image acq. time: 96 x 0.75ms = 72 ms (distortion!) TR = 1 s TE = 30ms (32 nd line) Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Image acq. time: ___________

Psy 8960, Fall ‘06 EPI, Part 2: variants15 K-space trajectory and scaled T 2 & T 2 * decay in gray matter: 128 (complex-valued) points T acq = 72 ms TE = 30 ms time (ms) MM … 96 lines T2*T2* T2T2

Psy 8960, Fall ‘06 EPI, Part 2: variants16 Image: GE EPI, long TE and read-outGE EPI, shorter TE and read-out

Psy 8960, Fall ‘06 EPI, Part 2: variants RF G SS G PE G RO DAC 128 … … (96 times) Echo spacing = 0.75ms Pulse sequence diagram: 90 deg. Sequence type: SE EPI (partial Fourier acquisition) Contrast: T 2 Image resolution: 128 x 128 Image acq. time: 96 x 0.75ms = 72 ms TR = 5 s TE = 60ms (32 nd line) 180 deg. Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Image acq. time: ___________

Psy 8960, Fall ‘06 EPI, Part 2: variants18 K-space trajectory and scaled T 2 & T 2 * decay in gray matter: 128 (complex-valued) points T acq = 72 ms TE = 60 ms time (ms) MM … 96 lines 180  90  T2*T2* T2T2

Psy 8960, Fall ‘06 EPI, Part 2: variants19 Image: SE, more superior GE SE, more inferior

Psy 8960, Fall ‘06 EPI, Part 2: variants20 RF G SS G PE G RO DAC … Echo spacing = 0.75ms Pulse sequence diagram: 90 deg. Sequence type: IR SE EPI (4-shot) Contrast: T 1 and (some) T 2 Image resolution: 128 x 128 Image acq. time: 4 x 2.5s = 10 s TE = 25ms (16 th line) 180  N rep = 4 TR = 2.5 s TI = 0.9 s 180  … … … Sequence type: ___________ Contrast: ______________________ Image resolution: ___________ Image acq. time: ___________ (32 times)

Psy 8960, Fall ‘06 EPI, Part 2: variants21 K-space trajectory and scaled T 2 * decay in gray matter: 128 (complex-valued) points … … … … 128 lines T acq,seg = 24ms TE = 40ms time (ms) MM 180  90  T2*T2* T 2, white matter T 2, gray matter

Psy 8960, Fall ‘06 EPI, Part 2: variants22 Image:

Psy 8960, Fall ‘06 EPI, Part 2: variants23 Pulse sequence diagram: N rep = 176 x points RF G SS G PE G RO DAC Sequence type: 3D FLASH (slab-selective excitation) Contrast: T 1 Image resolution: 256 x 224 x 176 Image acq. time: 176 x 224 x.01s = 6 min 34s Sequence type: ___________ Contrast: ______________________ Image resolution: ___________ Image acq. time: ___________ TR = 10ms

Psy 8960, Fall ‘06 EPI, Part 2: variants24 K-space trajectory and scaled T 2 * decay in gray matter: 256 (complex-valued) points T acq = 2.5ms TE = 5ms time (ms) MM 224 lines 176 partitions

Psy 8960, Fall ‘06 EPI, Part 2: variants25 Image: